Polybutylene terephthalate (PBT) is a well known crystalline thermoplastic engineering resin due to its excellent mechanical and electrical properties, as well as its physical and chemical characteristics. PBT is thus widely used as an engineering resin in a variety of fields including component parts for the automotive, electrical and electronic appliance industries, and the like.
PBT resins have conventionally been used in components for end-use applications which demand flame-retardant properties, such as molded parts for use with electrical and electronic components. In this regard, flame-retardant PBT compositions have typically been prepared by blending a PBT base resin with an organic halogenated flame retardant, such as decarbromodiphenyl ethers or brominated polycarbonate oligomers, either alone or with an inorganic auxiliary flame retardant such as antimony trioxide. However, such conventional flame-retardant PBT resin compositions are problematic when used in close association with an electrical contact of an electrical or electronic component (e.g. as occurs when the PBT composition is used as a support structure for the electrical contact). That is, conventional flame-retardant PBT resin compositions tend to generate a relatively large amount of gaseous compounds during long-term and/or high-temperature service which reduces the contact characteristics of the electrical component with which the composition is used.
The reasons why such "off-gassing" problems occur is presumed to be that, when conventional PBT compositions are heated during molding and/or when molded articles made from the compositions are used in high-temperature environments, the PBT base resin, flame retardants and/or decomposition products are carbonized virtue of a slight arc occurring with the electrical contact. As a result, these carbonized materials release gaseous compounds which are thought to adhere to the contact thereby promoting its corrosion.
In order to solve the problems noted above, brominated bisimides, brominated epoxy oligomers and the like have been proposed as flame retardants for PBT compositions. However these brominated flame retardants still yield insufficient results, even though PBT compositions containing them exhibit much less of an off-gassing problem. In addition, the use of brominated epoxy oligomers creates other problems in that the viscosity of the resulting composition increases during its molding residence time resulting in poor melt-flow characteristics.
Techniques to inhibit the thermal decomposition of PBT resins by adding phenols, amines or phosphorus compounds have been also proposed. However, PBT resin compositions containing such compounds do not exhibit satisfactory performance characteristics when used to support electrical contacts, even though such compositions generate reduced amounts of a gas in service.
What has been needed in the art, therefore, are flame-retardant PBT compositions which exhibit minimal (if any) off-gassing characteristics which could thus be used successfully as support components or the like for electrical contacts. It is toward fulfilling such a need that the present invention is directed.
Broadly, the present invention is embodied in moldable flame-retardant PBT resin compositions which include (A) a PBT resin, (B) a halogenated phenoxy compound having a number-average degree of polymerization of 6 to 30, (C) an alkali metal oxide-containing antimony pentaoxide compound, (D) a fatty acid ester of a polyhydric alcohol, and optionally (E) an inorganic filler material. More specifically, preferred embodiments of the present invention will include:
(A) 100 parts by weight of a polybutylene terephthalate base resin having an intrinsic viscosity of 0.75 to 1.5 dl/g;
(B) between 1 to 50 parts by weight of a halogenated phenoxy compound which, exclusive of chain-terminating groups, is composed of repeating units represented by the general formula (1) and having a number-average degree of polymerization of 6 to 30: ##STR1## wherein X represents a bromine or chlorine atom; and Y represents alkylene having 1 to 10 carbon atoms, alkylidene, cycloalkane, carbonyl, --O--, --S-- or --SO.sub.2 --;
(C) between 0.1 to 30 parts by weight of an antimony compound containing both antimony pentaoxide and an alkali metal oxide as constituents;
(D) between 0.01 to 10 parts by weight of a fatty acid ester of a polyhydric alcohol; and
(E) between 0 to 150 parts by weight of an inorganic filler material.